Investment Securities (Mortgage-backed, Asset-backed Securities) (Details) - USD ($) $ in Millions	Jun. 30, 2017	Dec. 31, 2016	Jun. 30, 2016	Dec. 31, 2015
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 2,633.8	$ 2,547.3	$ 2,604.6	$ 1,543.5
Non-Agency Residential Mortgage
Schedule of Trading Securities and Other Trading Assets [Line Items]
Fair value investments	$ 6.3